THE IMPACT OF COGENERATION IN A GIVEN ENERGETIC CONTEXT
Kris R. Voorspools, William D. D’haeseleer
University of Leuven (K.U.Leuven), Division of Applied Mechanics and Energy Conversion
Celestijnenlaan 300A / B-3001 Leuven / Belgium
In order to evaluate the value of cogeneration, mostly static and very simplified criteria are used,
only taking into account the thermal and electrical efficiency of both the cogeneration option and
the alternatives for separate production. This simple comparison neglects the entire energetic
context and the dynamic interaction between cogeneration and the local energy system. A very
important factor in the comparison between cogeneration and separate production is the equivalence
in final energy delivery of both options. The energetic equivalence manifests itself in the electric
and thermal power, the annual use of this power and the time period in which the demand is present.
Also the impact of the installation of cogeneration on the investment in additional central electricity
generation capacity is crucial in the comparison between the options with or without cogeneration.
Because the limitations of the static methods, a new approach, based on simulation of scenarios, is
developed. For a given demand for heat and electricity, two scenarios are worked out: a scenario
where no additional cogeneration is installed and a scenario where extra cogeneration is added,
possibly also resulting in a reduced expansion of the central power system. To correctly portray the
dynamic response of the central power system, the entire electricity production is simulated.
The use of this method for concrete possibilities for cogeneration in Belgium demonstrates the need
for this new dynamic method. For industrial cogeneration, the static simplified method seems to be
quite valid because of the high and constant utilization of this form of cogeneration. In the case of
cogeneration in the tertiary sector, where the heat demand is only present during a limited period of
time, the static method is not valid. As a general conclusion, it can be stated that every specific
possibility for cogeneration has to be evaluated separately in its overall energetic context, including
the entire electricity generation system.
EXISTING METHODS FOR EVALUATING COGENERATION
The value of cogeneration is usually evaluated by means of simple comparison schemes. Through a
careful examination of such schemes, we demonstrate that they are incomplete and may lead to
erroneous conclusions. From the identification of the shortcomings, the need for a more complete
approach will be obvious. That new approach will then be introduced in the next section.
Static simplified comparison
The scheme that is most frequently used is what we call the static simplified comparison of
cogeneration and separate generation of heat and electricity (see, e.g., Martens  and Program
SAVE ). This method is illustrated in Figure 1. On the left, a cogeneration unit produces heat
and electricity. On the right, the same amount of heat and electricity is generated separately.
COGENERATION SEPARATE PRODUCTION
FC αQ FS
Figure 1 : Simplified static comparison of cogeneration and separate generation of heat and power
Starting from the simple scheme of Figure 1, several expressions can be used to express the quality
of the cogeneration unit:
Saving in primary energy : E + Q − 1
(relative to the fuel usage in the cogeneration unit) E Q
The quality index QI : 1 −
Both the representation in Figure 1 and the equations above are very easy to understand and seem
logical because in one case the heat and the power are delivered by cogeneration, and in the other
case by separate production units. If one looks more closely, however, this comparison between
cogeneration and separate production of heat and electricity is less obvious.
Since no time aspects are taken into account, the equivalence between both options has to be
instantaneous. This means that both systems deliver the same power (in MW) but not necessarily
the same amount of energy (in MWh). If the use of both systems is not simultaneous (and this is all
but self-evident), there is no energetic equivalence. E.g., if a combined-cycle (CC) gas-fired unit is
chosen for electricity generation, its annual use will be about 7000 h/a (or 80%). This is
comparable to the annual use of industrial cogeneration, but is much larger than the annual use of
cogeneration in the tertiary or the domestic sectors (about 4000 h/a or 40 to 50%). An evident
question here is what happens during the time that the cogeneration units are not active. The CC
gas-fired plant in question cannot be used because it has not been built… Although both options
(CC gas-fired plant versus cogeneration) are able to deliver the same power, they do not generate
the same amount of electricity. This gap has to be filled by other means which are not mentioned in
this static simplified method.
Also, the possibility that one of the systems (cogeneration, power plant or furnace) operates at
partial load with an efficiency lower than the nominal efficiency, is usually not taken into account
with this static simplified method. Furthermore, the need for back-up furnaces in the case where the
cogeneration unit itself cannot deliver all the heat required, is not accounted for.
Static comparison including overall operation time
To meet the objection that no time duration aspects are taken into account in the static simplified
method, attempts have been made to statically include time by looking at the annual use of the
technologies considered (Martens ). The method is illustrated in Figure 2. Again, the situations
with cogeneration and with the separate production of electricity and heat, are compared. Now also
the annual operation time is taken into account. In the case where cogeneration is installed, it
produces heat and power. The remainder of the time other production units will have to take over
electricity generation ('back-up' power). In the case of separate production, furnaces deliver the
heat and a reference electric power plant delivers electricity. Here also, back-up power takes over
when this reference plant is inactive.
COGENERATION SEPARATE PRODUCTION
’back-up’ reference ’back-up’
E power plant E power plant power plant
Figure 2 : Static comparison of cogeneration and separate generation of heat and power,
including overall operation time
Using the representation in Figure 2, the following condition for energy saving can be derived (U
stand for the total annual use of a system).
E ⋅ U plant (
E ⋅ 8760 − U plant ) E ⋅ U cogen (
E ⋅ 8760 − U cogen )
Q ⋅ U heat
+ + > +
plant back−up furnace E back−up
In this formula the expression on the left represents the energy use for separate production. The
terms respectively stand for the energy use of the power plant, the back-up power plant and the
furnace. The expression on the right stands for the energy use in the case of cogeneration and the
terms respectively stand for the energy use of the cogeneration unit and the back-up power plant.
E and Q represent the electric and thermal power.
Although this approach apparently deals with the main shortcoming of the static simplified method
discussed earlier, it does not quite. In the formula, as well as in Figure 2, a choice has to be made
for the backing power plant used at times where the cogeneration unit or the reference power plant
are inactive. In reality, there is no power plant that is "reserved" for this purpose, but additional
power will be delivered by a variable combination of plants still available in the power system. If
possible, already activated plants can be modulated to deliver this power. If all plants already
operate at maximum power level, new plants will have to be started.
Also, the dynamic response of the central power system is completely neglected by this static
comparison, because only the integrated operation time of all systems is considered. The reaction
of the central power system depends on the instantaneous demand, not on the annual demand. If the
hours in which additional power is required are consecutive, a large power station may be used to
cover this demand. If not, the fluctuation will probably be leveled with peak power plants or
The only possibility to get some kind of results from this method is to perform a parametric analysis
concerning all possibilities for the additional power needs, ranging from the modulation of very
efficient plants to the start-up of a less efficient plant. This will however not lead to concrete results
but a large range of results. Interpretation of these results may be an interesting academic exercise,
but is quite useless for drawing conclusions.
More sophisticated static methods
In order to deal with some of the objections stated above, more sophisticated methods could be
devised. It is, however, by definition impossible to include dynamic considerations (such as the
time dependent response of the central power system to instantaneous fluctuations in demand) in a
static formulation. Since most of the objections stated above are also connected to these dynamic
considerations, it is of little practical use to devise more sophisticated static methods.
MODELLING OF COGENERATION
To take into account dynamic consideration, the entire energetic (electricity and heating) context is
simulated on a sufficiently small time scale. In our approach we consider an hourly time grid. For
heating, the hourly heat demand of the application or sector considered is used. For electricity, the
model PROMIX (Voorspools and D’haeseleer ,) is used to dynamically simulated the hourly
To correctly evaluate the impact of cogeneration, the approach illustrated in Figure 3 is used. Two
future situations are compared. In one situation being the result of some reference scenario, say
2000-2010, electricity demand has been projected, accompanied with a "natural" expansion of the
central power system. The other situation is identical to the first one, except that an additional
amount of cogeneration is postulated. In this second situation, care has to be taken that the entire
energetic context is evaluated. If this additional cogeneration prevents the installation of new
central power units, this also has to be accounted for in the simulation. The boundary condition
linking both situations is the equivalence in final energy delivery (electricity and heat). Thus, the
differences between both situations (energy use and emissions) are ascribed to the extra
SITUATION WITHOUT SITUATION WITH
EXTRA COGENERATION EXTRA COGENERATION
CENTRAL simulation ELECTRICITY simulation CENTRAL
POWER SYSTEM model DEMAND model POWER SYSTEM
context without cogeneration context with cogeneration
FURNACES IN THE HEAT DEMAND IN THE COGENERATION IN THE
SPECIFIED SECTOR SPECIFIED SECTOR SPECIFIED SECTOR
ADDITIONAL FURNACE IN
THE SPECIFIED SECTOR
ENERGY USE ∆ ENERGY USE ENERGY USE
EMISSIONS ∆ EMISSIONS EMISSIONS
situation without cogeneration responsibility of cogeneration situation with cogeneration
Figure 3 : Method for the evaluation of cogeneration
In order to demonstrate the importance of the dynamic simulation of cogeneration in the entire
energetic context, two applications are considered. In the first cases study, industrial cogeneration
is simulated, in the second, we consider cogeneration in the tertiary sector (administrative and
commercial buildings). In both cases, the scenario approach illustrated in Figure 3 is used. The
time horizon of all scenarios is 2010 and the energetic context is Belgian.
By 2010, about 1700 MWel of industrial cogeneration will already be present in the reference
scenario of our simulations. In the case study on industrial cogeneration, an additional 360 MWel of
cogeneration is postulated. Because the heat demand relevant to this case study is relatively
constant, cogeneration units can be dimensioned to cover it almost entirely. The average annual use
of these cogeneration units in about 7500 h/a, or 85%.
Using the approach illustrated in Figure 3, two situations are simulated. In the first no additional
industrial cogeneration is simulated. In the second, 360 MWel of industrial cogeneration is added.
Because this cogeneration lowers the demand peak for the central power system, also the
investment strategy in central power will be altered. In the current energetic context (projected on
to 2010 in our scenarios) this means that one CC gas-fired unit of 400 MWel (reliability of 90%)
will not be built in the period considered.
Comparison of both scenarios in Table 1 shows that an additional installation of 360 MWel of
industrial cogeneration is responsible for an overall reduction in primary energy use and
greenhouse-gas emissions of respectively 4000 TJ and 250 kton.
These results compare very well with the results of the static simplified comparison (4000 TJ and
240 kton). In this specific case, this simplified method is valid. Indeed, none of the objections
stated in the explanation of the static simplified method seem to apply here. The annual use of the
cogeneration units approaches the annual use of the CC gas-fired unit and all systems continuously
operate at full load.
Table 1 : Evaluation of 360 MWel industrial cogeneration
360 MWel additional
Time horizon 2010 reference
central 94 950 92 240
electricity generation [GWhel]
additional cogeneration 0 2 710
furnaces 2 990 0
relevant heat production [GWhth]
cogeneration 0 2 990
central electric 815 600 798 000
furnaces 12 000 0
primary energy use [TJ]
additional cogeneration 0 25 600
saving —— 4 000
central electric 26 400 25 400
furnaces 700 0
greenhouse-gas emissions [ktonCO2-eq]
additional cogeneration 0 1 500
reduction —— 250
energy saving [TJ] 4 000
reduction in emissions [ktonCO2-eq] 240
Cogeneration in the tertiary sector
In the second case study, 360 MWel of cogeneration in the tertiary sector is supposed to be installed.
Since most of the heat demand in the tertiary sector is used for heating of buildings, its annual use is
low (about 4000 h/a or 45%). Also, because the heat demand strongly fluctuates, the cogeneration
units will be dimensioned to cover a more constant base load. The peaks and the fluctuation are
still covered by furnaces. In Figure 4, the heat demand in this sector is plotted together with the
heat production from the cogeneration units (Luyckx and Martens ). The installation of this
cogeneration will also interact with the investment strategy of the central power system leading to
the avoidance of the construction of one CC gas-fired unit of 400 MWel.
heat demand sector
heat production cogeneration
heat demand [MWth]
Figure 4 : Heat demand in the tertiary sector and heat production from cogeneration
In Table 2 the results of the simulations with and without 360 MWel of extra cogeneration in the
tertiary sector by 2010 are shown. Comparison of both scenarios shows that here the cogeneration
is responsible for an overall reduction in primary energy use and greenhouse-gas emissions of
respectively only 1800 TJ and 30 kton.
The results for the simplified static comparison are much more favorable, namely a reduction in
energy use and greenhouse-gas emissions of respectively 2900 TJ and 170 kton. It is clear that the
static method largely overestimates the potential of this type of cogeneration. Looking at the
objections to the static method, we see that the main shortcoming here is that the annual use of these
cogeneration units is much smaller than that of the avoided CC gas-fired plant would have been,
namely 4000 versus 7000 h/a. Therefore, both systems are not electric equivalents and the available
part of the central power system will have to match the difference. Looking more closely at the
results, we find that this additional power mainly comes from coal fired power plants: the electricity
generation of the CC gas-fired units decreases from 32 110 to 30 450 GWh (which is logical
because one less CC gas-fired unit is built), whereas the generation of the coal fired units rises from
2 600 to 2 830 GWh. Since these plants are less efficient and emit much more greenhouse-gases,
the result from the dynamic simulation is much less favorable than predicted by the static method.
Table 2 : Evaluation of 360 MWel cogeneration in the tertiary sector
360 MWel additional
Time horizon 2010 reference cogeneration
in the tertiary sector
central 94 950 93 490
electricity generation [GWhel]
additional cogeneration 0 1 460
furnaces 4 630 2 540
relevant heat production [GWhth]
cogeneration 0 2 080
central electric 815 600 807 300
furnaces 19 100 10 700
primary energy use [TJ]
additional cogeneration 0 15 000
saving —— 1 800
central electric 26 400 26 000
furnaces 1 100 600
greenhouse-gas emissions [ktonCO2-eq]
additional cogeneration 0 900
reduction —— 30
energy saving [TJ] 2 900
reduction in emissions [ktonCO2-eq] 170
For the evaluation of the potential in energy saving and emission reduction of cogeneration, static
methods are commonly used. They are, however, unable to take into account the dynamic response
of the energy system. For this reason, a new approach is developed in which the entire electricity
generating system as well as the specified heat delivery are simulated. For every specific case, two
scenarios are worked out. In one scenario, the heat is delivered by furnaces. In the other scenario
cogeneration units are fitted into this heat demand. In both scenarios, the total energy delivery
(central units and cogeneration units) remains unchanged. The central power generation is
simulated on an hourly basis to fully reckon with both short as long term dynamic parameters.
Concrete simulations demonstrate the discrepancy between the results of the static methods and the
dynamic simulations. For industrial cogeneration with a high and constant utilization, the results of
the static method correspond quite well to those of the simulations. For cogeneration in the tertiary
sector, on the other hand, the results of the static method are much more optimistic than those of the
simulations. This is caused by the sub-optimal use of the central power system and the increased
share of less optimal power stations due to the installation and especially the limited use of this kind
1. Martens A. Energetische rentabiliteit van WKK (Energetic return of cogeneration).
VITO Report # REG.RV9604, VITO, Mol (Belgium), June 1996.
2. Program SAVE. La petite cogénération. Pourquoi? Pour qui? (Small cogeneration. Why? For
whom?). Ministry Wallonia, Namur (Belgium), March 1997.
3. Voorspools K., D’haeseleer W. The influence of the instantaneous fuel mix for electricity
generation on the corresponding emissions. Energy, accepted for publication; in print.
4. Voorspools K., D’haeseleer W. An evaluation method for calculating the emission
responsibility of specific electric applications. Energy Policy, accepted for publication, in print.
5. Luyckx W., Martens A. WKK, STEG, kolencentrale en hun CO2 equivalente emissies
(Cogeneration, CC gas-fired plants, coal fired plants and their CO2 equivalent emissions).
VITO Report # 2000/ETE/R/008, VITO, Mol (Belgium), January 2000.